Streptococcus suis is an important swine pathogen that causes many pathological conditions such as arthritis, endocarditis, meningitis, pneumonia and septicemia (13, 14). It is also an important zoonotic agent for people in contact with contaminated pigs or their by-products, causing meningitis and endocarditis (1, 36). Thirty-three serotypes (types 1 to 31, 33 and 1/2) based on capsular antigens are currently known (9-11, 15, 17, 31). Type 2 is considered the most virulent and prevalent type in diseased pigs. The mechanisms involved in the pathogenesis and virulence of S. suis are not completely understood (13) and attempts to control the infection are hampered by the lack of effective vaccines.
Several approaches have been made to develop vaccines for S. suis. However, little success was achieved because the protection was either serotype or strain dependent and results, in most instances, were equivocal (16, 30). For example, some protection with killed whole cells and live avirulent vaccines were reported, but this required repeated immunization and the protection against heterologous challenges was not determined (18, 38). Exposure of young pigs with live virulent strains showed a positive effect in reducing clinical signs characteristics of S. suis infection, but not in central nervous sign and mortality (35). Since the S. suis capsule plays an important role in virulence, attempts have been made to develop a vaccine based on capsular material. However, this vaccination was unsatisfactory because the capsular polysaccharide is poorly immunogenic (7). More recently, interest has shifted toward protein antigens of S. suis as vaccine candidates. Subunit vaccines using suilysin (20), or MRP (muramidase-released protein) and EF (extracellular proteins factor) (39) have been shown to protect pigs from homologous and heterologous serotype 2 strains, but their use is hindered by the fact that a substantial number of the virulent strains in some geographical regions do not express these proteins (8, 12, 29). Thus, identification of other antigenic factors, especially surface proteins, could contribute to the development of a subunit vaccine.
There is thus a need for the discovery and use of new targets for the prevention, the treatment and the diagnosis of S. suis-associated diseases and infections caused by S. suis.